1. Field of the Invention
The present invention is directed generally to a glass blank used as a molding material in press molding of an optical element such as a lens or the like, and more particularly, to a glass blank for obtaining a favorable optical element by preventing cracking in a cooling process as well as by reducing frictional force and adhesion to mold members during a pressing operation.
2. Related Background Art
Preventing fusion between a molding material glass and molding mold members has hitherto been a big problem in terms of obtaining a good molded product in a reheat press of the glass. For this purpose, a variety of techniques for improving the mold members were proposed. In recent years, in order to prevent the fusion the further proposals of improvements of the molding materials have begun to appear. Proposals on the improvements of such molding materials were disclosed in, for example, Japanese Patent Publication Nos. 2-1778, 2-1779, 2-1780 and 61-29890. In these publications it is disclosed that the surface of the glass substrate is coated with a glass layer, a silicon oxide layer or carbon layer each having a higher glass transition point temperature than that of the glass substrate. Disclosed in Japanese Patent Laid-Open Application No. 1-264937 is such an arrangement that a thin layer of an organic substance is coated on the surface of the glass substrate.
The improvements permit to obtain an effect for preventing the fusion to the mold members. There arise, however, the following problems inherent in the methods disclosed in the above-mentioned Patent Publications.
(a) Where the glass layer having a higher glass transition point temperature than that of the glass substrate is coated on the surface of the glass substrate, the glass layer coated thereon is cracked due to a press pressure. The substrate glass oozes out from the crack to cause partial fog on the surface or partial fusion to the mold members. Alternatively, the frictional force between the glass blank and the mold members increases during a cooling process, with the result that the glass may be cracked in some cases.
(b) Where the silicon oxide layer is coated on the surface of the glass substrate, as in the case of (a), the crack is liable to occur during the cooling process because of a good matching property of silicon oxide to the mold members. A thermal expansion coefficient of silicon oxide is remarkably higher than that of a typical optical glass which constitutes the glass substrate. Hence, the silicon oxide layer tends to crack during a heating process.
(c) Where the carbon layer is coated thicker on the surface of the glass substrate than required, the carbon reacts to oxygen in the glass, because carbon is a reducing agent. The glass component is thereby reduced, so that the glass is colored in brown. Especially, when a lead-contained glass is employed as a glass substrate. PbO in the glass is reduced. As a result, coloring becomes conspicuous and its transmissivity deteriorates.
(d) Where the organic substance layer is coated on the surface of the glass substrate, the organic substance layer is decomposed during heating. In some cases a corrosive gas (e.g., a chloric gas and a fluoric gas) is generated to thereby cause a problem in which the press molding device is contaminated, and a durability of the device including the mold members is deteriorated. The decomposition described above partially takes place at random, and hence a surface accuracy may decline.